In drilling an oil well, a drill string having a drill bit affixed to the bottom is customarily used. The drill string is connected into a mud flow system which typically utilizes drilling mud which is pumped by a multiple cylinder pump. The pump connects through a mud line to the top of the drill string, and the mud is delivered under pressure to the top of the drill string for flowing through the drill string to the drill bit. The pump typically operates at high pressures, and pressures in the range of 2,000 to 3,000 psi at the pump discharge outlet are not uncommon. Typical pumps are multiple cylinder pumps. During the operation of the pumps, there are pressure surges in the range of 200 to 300 psi which are caused by power strokes of individual pistons within the pump. These pressure surges are quite large, especially at the surface where there is a minimum of pressure surge damping in the mud delivery system.
Measuring while drilling apparatus has been known heretofore. Such equipment typically operates by forming variable constrictions in the drill string. This forms a pressure pulse transferred through the standing column of mud in the drill string back to the surface. As an example, a constriction might be formed by the measuring while drilling apparatus which signal is then coupled through 10,000 feet of mud standing as a column within the drill pipe. While the mud might be deemed to be an incompressible fluid, nevertheless, the signal received at the surface from the downhole equipment is relatively small. It is relatively small, smaller than the pump surges found at the surface. The pressure pulses or variations at the surface are large; they can be ten to one hundred times greater than the variable data from the measuring while drilling apparatus.
In the operation of measuring while drilling apparatus, pressure pulses travel through the mud at a velocity equal to the acoustic velocity of the medium. Depending on the makeup of the drilling mud, this is a velocity of about 4,000 to 5,000 feet per second. Moreover, each pressure pulse is accompanied by a change in fluid velocity which is defined by the relationship quantifying the water hammer effect. The pressure change-velocity change relationship is therefore given by the following equation: EQU .DELTA.P=R.sub.0 C.DELTA.V (1)
where
.DELTA.P=the magnitude of the pressure pulse, PA1 R.sub.0 =the fluid mass density, PA1 C=the acoustic velocity in the fluid, and PA1 .DELTA.V=the change in fluid velocity. PA1 .DELTA.V=the velocity variation, PA1 .DELTA.P=the pressure variation, PA1 R.sub.0 =the fluid density, and PA1 C, K.sub.1 and K.sub.2 =various constants.
As will be understood from the foregoing equation, pressure pulses formed by measuring while drilling apparatus are related to fluid velocity changes in the foregoing equation.
A typical mud pump forms a pressure surge during the power stroke of the individual pistons in it. This represents a positive pressure surge. This increases the mud flow velocity in the drill string. Conversely, pressure pulses from measuring while drilling apparatus located downhole decrease the mud flow velocity as a result of propogation in the opposite direction. Taking into account the direction of propogation in the system, there is, therefore, an interesting relationship. For a given pressure surge originating with the pump and moving downstream in the same direction as the mud flow, there is a positive pressure increase and a related velocity increase. Conversely, where the pressure pulse is originated at the downhole equipment and moves upstream against the flow of mud, a pulse originating at measuring while drilling apparatus and propogated upstream against the flow of mud is accompanied by a decrease in velocity of mud in the drill string. Intuitively, this conforms to the observation that measuring while drilling apparatus which momentarily constricts the mud flow to form a pressure pulse also retards the mud flow velocity.
This is a linear phenomena, and, thus, pulses traveling in both directions add algebraically. Pressure and velocity variations are thus cumulative.
From the foregoing, it will be observed that the ultrasonic fluid velocity is modified by pressure surges traveling both ways. It will be appreciated that pressure surges add to or momentarily reduce the quiescent pressure which is maintained in the system. As an example, the quiescent pressure may be 3,000 psi, whil the pump surges may be 200 or 300 psi short peaks added to the quiescent pressure. In any case, the quiescent pressure can be likened to a base value which, if properly manipulated, can be removed so that only pressure variations are noted.
The transient portion of the pressure signal is proportional to the flow or velocity variation. Thus, it is possible to obtain a signal which is proportional to pressure variations. It is also possible to obtain a signal proportional to velocity variations. These two signals are given by the following relationships: EQU S.DELTA.V=K.sub.1 .DELTA.V (2) EQU S.DELTA.P=K.sub.2 .DELTA.P (3)
A fundamental relationship which exists is given by the equation: EQU .DELTA.P=R.sub.0 C.DELTA.V (4)
where
Substituting equations, one obtains the relationship of: EQU S.DELTA.P=K.sub.3 S.DELTA.V (5)
Through the manipulation of signals representative of these two quantities, reversal of polarity and appropriate addition, they can be offset against one another to obtain a null value. This relationship would thus be true in the case of positive values of pressure variation (increases above the quiescent value). This apparatus thus utilizes the detector system disclosed herein to obtain the difference in sensor outputs (after appropriate signal modification) which are caused by pump pressure and velocity variations and yields a null output. When this occurs, pump pressure pulses (noise) are eliminated from the output signal.
When a pressure variation is received from the downhole measuring while drilling apparatus, it is additive because the downhole pressure variation (accompanied by a propogated velocity variation) passes through the detector apparatus which responds to the velocity variation as a drop in velocity. In other words, a pulse propogated from the measuring while drilling apparatus has two manifestations, one being a pressure increase at the measuring equipment and the other being a velocity drop. In light of the relative polarity of the two signals which are created by the present apparatus, pump pressure pulses are eliminated, while pressure pulses from the downhole equipment are markedly increased.
The effects mentioned above are linear and, therefore, additive. Being additive, the pump noise (having the form of pressure and velocity spikes) are eliminated, while the signals from the downhole equipment are output by the equipment. The output signal is enhanced or increased.